Combustion staging system

ABSTRACT

A combustion staging system is provided for fuel injectors of a multi-stage combustor of a gas turbine engine. The system has a splitting unit which receives a metered total fuel flow and controllably splits the metered total fuel flow into out-going pilot and mains fuel flows to perform pilot-only and pilot-and-mains staging control of the combustor. The system further has pilot and mains fuel manifolds which respectively receive the pilot and mains fuel flows, the mains fuel manifold being split into a primary line and a servo line such that each line receives a respective portion of the mains fuel flow. The system further has a plurality of mains flow control valves which distribute the mains fuel flow from the mains fuel manifold to mains discharge orifices of respective injectors of the combustor, both the primary line and the servo line extending to the mains flow control valves before reuniting. The system further has a servo pump operable to change the pressure in the servo line relative to the pressure in the primary line. Each mains flow control valve has a chamber containing a movable piston, the chamber to a primary side of the piston being fed by the primary line, and the chamber to a servo side of the piston being fed by the servo line. The piston is biased towards a closed pilot-only position which prevents flow out of the primary side of the chamber to the mains discharge orifice of the respective injector. The piston is movable under an increase in pressure in the servo line relative to the primary line to an open pilot-and-mains position which allows flow out of the primary side of the chamber to the mains discharge orifice of the respective injector.

FIELD OF THE INVENTION

The present invention relates to a combustion staging system for fuelinjectors of a multi-stage combustor of a gas turbine engine.

BACKGROUND

Multi-stage combustors are used particularly in lean burn fuel systemsof gas turbine engines to reduce unwanted emissions while maintainingthermal efficiency and flame stability. For example, duplex fuelinjectors have pilot and mains fuel manifolds feeding pilot and mainsdischarge orifices of the injectors. At low power conditions only thepilot stage is activated, while at higher power conditions both pilotand mains stages are activated. The fuel for the manifolds typicallyderives from a pumped and metered supply. A splitter valve can then beprovided to selectively split the metered supply between the manifoldsas required for a given staging condition.

A typical annular combustor has a circumferential arrangement of fuelinjectors, each associated with respective pilot and mains feedsextending from the circumferentially extending pilot and mainsmanifolds. Each injector generally has a nozzle forming the dischargeorifices which discharge fuel into the combustion chamber of thecombustor, a feed arm for the transport of fuel to the nozzle, and ahead at the outside of the combustor at which the pilot and mains feedsenter the feed arm. Within the injectors, a check valve, known as a flowscheduling valve (FSV), is typically associated with each feed in orderto retain a primed manifold when de-staged and at shut-down. The FSVsalso prevent fuel flow into the injector nozzle when the supply pressureis less than the cracking pressure (i.e. less than a given differencebetween manifold pressure and combustor gas pressure).

Multi-stage combustors may have further stages and/or manifolds. Forexample, the pilot manifold may be split into two manifolds for leanblow-out prevention during rapid engine decelerations.

During pilot-only operation, the splitter valve directs fuel for burnerflow only through the pilot fuel circuit (i.e. pilot manifold andfeeds). It is therefore conventional to control temperatures in thede-staged (i.e. mains) fuel circuit to prevent coking due to heat pickup from the hot engine casing. One known approach, for example describedin EP A 2469057, is to provide a separate recirculation manifold whichis used to keep the fuel in the mains manifold cool when it isdeselected. It does this by keeping the fuel in the mains manifoldmoving, although a cooling flow also has to be maintained in therecirculation manifold during mains operation to avoid coking.

However, a problem with such a system is how to accommodate a mains FSVfailing to an open condition. In pilot-only operation, when cooling flowis passing through the recirculation manifold and the mains manifold,such a failure can result in the cooling flow passing through the failedopen FSV through one injector into the combustor, causing a hot streakwhich may lead to nozzle and turbine damage. In pilot and mainsoperation, such a failure can produce a drop in mains manifold pressurewhich causes other mains FSVs to close. A possible outcome is again thata high proportion of the total mains flow passes through the failed openFSV to one injector, causing a hot streak leading to nozzle and turbinedamage.

In principle, such failure modes can be detected by appropriatethermocouple arrangements, e.g. to detect hot streaks. However,temperature measurement devices of this type can themselves havereliability issues.

Further, the problem of mains FSV failure can be exacerbated by systemarrangements used to prevent combustion chamber gas ingress through thefuel injectors during pilot only operation. Whilst the impact of suchgas ingress is generally non-hazardous, it can lead to hot gas-induceddegradation of FSV seals. Degraded FSV sealing can in turn lead todribbling of fuel into de-staged nozzles, resulting in componentblockage due to coking.

US 2016/0273775 proposes a fuel staging system (reproduced in FIG. 1)that addresses some of the above problems. The staging system splits thefuel under the control of the engine electronic controller (EEC—notshown) into two flows: one at a pressure PB_(p1) for first 131 andsecond 132 pilot manifolds and the other at a pressure P_(fsv) for amains manifold 133. The first pilot manifold feeds pilot dischargeorifices of a subset of the fuel injectors. The second pilot manifoldfeeds pilot discharge orifices of the rest of the fuel injectors. Themains manifold feeds mains discharge orifices of all the fuel injectors.Mains fuel flow scheduling valves (FSVs) 140 at the injectors preventcombustion chamber gases entering the respective manifolds and alsoprovide a drip tight seal between the mains manifold and the injectorswhen mains is de-staged. By varying the fuel split between themanifolds, the EEC can thus perform staging control of the engine.

In more detail, the staging system 130 has a fuel flow splitting valve(FFSV) 135, which receives a metered fuel flow from a hydro-mechanicalunit (HMU) of the engine at pressure P_(fmu). A spool is slidable withinthe FFSV under the control of a servo-valve 146, the position of thespool determining the outgoing flow split between a pilot connectionpipe 136 which delivers fuel to the first 131 and second 132 pilotmanifolds and a mains connection pipe 137 which delivers fuel to themains manifold 133. The spool can be positioned so that the mains stageis deselected, with the entire metered flow going to the pilot stage(except that a cooling flow is sent to the mains manifold duringpilot-only operation, as discussed in more detail below). An LVDT (notshown) can provide feedback on the position of the spool to the EEC,which in turn controls staging by control of the servo-valve 146.

The pilot discharge orifices are divided into two groups by the first131 and second 132 pilot manifolds in order to provide lean blow outprotection. More particularly, the second pilot manifold connects to thepilot connection pipe 136 via a further connection pipe 139 (at apressure PB_(p2)) and a lean blow out protection valve 141. This isoperable to terminate or substantially reduce the supply of fuel to thesecond pilot manifold and associated pilot discharge orifices, whendesired, so as to increase the flow of fuel to the first pilot manifoldand associated discharge orifices under low fuel conditions for a givenmetered flow from the HMU. In the arrangement illustrated, the valve 141is controlled by way of a solenoid operated control valve 142, althoughother forms of control are possible, such as by a servo-type valve (forexample an electro hydraulic servo-valve). In this way, under low fuelconditions the flow of fuel to the pilot discharge orifices may bedirected preferentially via the first pilot manifold, whereby the riskof a lean blow out condition arising can be reduced. Further details ofsuch lean blow out protection are described in EP A 2469057.

The part of the staging system 130 comprising the FFSV 135, servo-valve146, lean blow out protection valve 141 and control valve 142 may behoused in a staging unit mounted to a fan case of the engine. Theconnection pipes 136, 137, 139 then extend across a bypass duct of theengine to the manifolds 131, 132, 133, which wrap around the core enginein proximity to the injectors 134. Alternatively, the staging system canbe mounted to the engine core.

In the staging system described in EP A 2469057, each injector has apilot FSV and a mains FSV for respectively the flows from pilot andmains manifolds. In contrast, in the staging system shown in FIG. 1,pilot FSVs are not necessary (although optional pilot FSVs can belocated between the mains FSVs 140 and the pilot discharge orifices),and instead pilot flow is routed through modified mains FSVs 140 withnegligible restriction: the mains FSVs 140 distribute the mains flowfrom the mains manifold 133 to the mains discharge orifices in theinjectors 134, while the pilot flow is passed through the mains FSVs forvalve cooling purposes. These FSVs each have a chamber containing amovable, spring-biased piston, with the chamber to a pilot (spring) sideof the piston being in fluid communication with the respective pilotfuel manifold 131, 132 and the chamber to a mains (non-spring) side ofthe piston being in fluid communication with the mains fuel manifold133. In this way, the FSVs 140 have a reduced cracking pressure with thepilot (spring) side of the FSVs being referenced to pilot manifoldpressure (PB_(p1) or PB_(p2)) rather than the lower mains pressuredownstream of the FSVs (as is the case with the system of EP A 2469057).With the low cracking pressure, the pressures on either side of eachpiston (P_(fsv) and PB_(p)) are approximately equal during thepilot-only operating mode such that the FSV springs maintain the FSVs140 in a closed position (i.e. no flow from the mains manifold 133through the FSVs to the mains discharge orifices of the injectors 134).The approximate equalisation of the pressures P_(fsv) and P_(bp) isachieved by energising open a single-stage solenoid-operated mainscooling valve 147 which allows a small cooling flow through the mainsmanifold 133 to pass to the second pilot manifold 132 through a port inthe solenoid valve 147 that is significantly larger than the port in theFFSV 135 feeding the mains connection pipe 137.

In this pilot-only operating mode, the position of the FFSV 135,controlled by the servo-valve 146, is such that there is a large flownumber opening between the HMU supply and the pilot connection pipe 136,such that P_(fmu)≈PB_(p1)≈P_(fsv). Any difference between the meteredfuel pressure (P_(fmu)) from the HMU supply and the pilot manifoldpressures (PB_(p1) and PB_(p2)) is insufficient to open the FSVs 140. Inthe pilot-only mode there is a small opening in the FFSV between the HMUsupply and the mains connection pipe 137 to allow for the cooling flowin the mains manifold 133. The mains manifold remains fully primed inpilot-only mode to reduce the unprimed volume required to be filled whenmains flow to the combustor is required. When mains staging is selectedsolenoid-operated mains cooling valve 147 is closed so that theconnection between the mains manifold 133 and the second pilot manifold132 is closed. Simultaneously, the FFSV 135 (controlled by theservo-valve 146) moves to increase the opening between the HMU supplyand the mains connection pipe 137. This reduces PB_(p1) and PB_(p2)relative to P_(fsv), resulting in fuel flow to the mains dischargeorifices of the injectors 134.

If one of the FSVs 140 fails such that it opens in pilot-and-mains mode,fuel flows from the HMU supply through the FFSV 135 to the mainsmanifold 133 and thence through the open port in the failed FSV to themains discharge orifice of the respective injector 134. However, as theFSVs have a relatively low cracking pressure, only a marginal increasein pressure in the mains manifold, resulting from flow through the portin the failed FSV, causes the other FSVs to open. This then leads to arelatively even distribution of fuel flow injection around thecombustor. Thus, by ensuring that the other FSVs open before a severelevel of fuel flow through the failed FSV is reached (i.e. a level thatresults in hot streaks and turbine damage), the staging system 130 canmitigate the potentially hazardous mal-distribution issues associatedwith failed open mains FSVs in the system of EP A 2469057. The latterincorporates high cracking pressure FSVs, potentially allowing a highlevel of flow to pass through a single failed open FSV (i.e. grossmaldistribution) before the other FSVs crack open.

The staging system 130 also allows complex cooling recirculationarchitectures to be avoided, which avoids the hazards that can resultfrom combustion gases leaking past mains FSVs and thence to the lowpressure side of the fuel system of the system.

Cooling of the FSVs 140 can be provided by the pilot flow that iscontinuously routed through the FSVs. Cooling arrangements can beprovided for the pilot manifolds 131, 132 and the mains 133 manifold,e.g. by using a small portion of the air flow through a bypass duct ofthe engine, and for the mains manifold in pilot-only operation using thecooling flow discussed below.

The pilot/mains flow split is achieved via movement of the spool withinthe FFSV 135, with a mains fuel flow sensing valve (MFFSV) 143 beingprovided on the mains connection pipe 137. The FFSV 135 then provides acoarse split and the MFFSV trims to the required accuracy. The positionof the FFSV 135 is controlled via the servo-valve 146 using the positionfeedback signal from the LVDT 144 attached to the MFFSV 143 to giveaccurate flow control in the connection pipes 136 and 137. Inparticular, the position feedback signal that is input to the stagingcontrol logic in the EEC is taken from an LVDT 144 measuring a spoolposition of the MFFSV rather than a spool position of the FFSV. In suchan arrangement, MFFSV spool position is a measure of the mains flow.

To provide the cooling flow in the mains manifold 133 during pilot-onlyoperation, the single-stage solenoid-operated mains cooling valve 147opens a bypass connection between the mains 133 and second pilot 132fuel manifolds, allowing the cooling flow to pass from the mains fuelmanifold to the pilot fuel manifold, and thence onwards for burning atthe pilot orifices of the injectors 134. The mains cooling valve 147closes during pilot-and-mains operation. The mains cooling valve has arelatively large minimum orifice size, and thus is relativelyinsensitive to contamination and ice build-up.

In the pilot-only operating mode, the cooling flow of fuel passescontinuously from the mains manifold 133 to the second pilot fuelmanifold 132, which maintains cooling in the mains manifold. Thiscooling flow is sensed by the MFFSV 143 and the feedback signal from theMFFSV LVDT 144 to the EEC is used to adjust the spool position of theFFSV 135 (via the servo-valve 146) if the cooling flow needs to bealtered. In pilot-only operating mode, even with the cooling flow thepressure drop across each FSV piston (P_(fsv)−PB_(p)) is insufficient toopen the FSVs.

When the pilot-and-mains operating mode is selected, the solenoidoperated mains cooling valve 147 is closed, and the spool position ofthe FFSV 135 is altered to increase the opening of the mains port of theFFSV and reduce the opening of the pilot port of the FFSV, whichincreases the pressure differential P_(fmu)−PB_(p1) across the pilotport, thus producing a rise in pressure P_(fsv) relative to PB_(p1) andPB_(p2). This results in the pistons of the FSVs 140 opening againsttheir respective spring forces, and fuel flowing through the FSVs to themains discharge orifices of the injectors 134. The MFFSV 143 now sensesthe flow to the mains discharge orifices of the injectors and thefeedback signal from the LVDT 144 is used to adjust the FFSV spoolposition via the EEC and FFSV servo-valve 146 to set the correctpilot/mains flow split.

Thus inclusion of the MFFSV 143 on the mains connection pipe 137 enablesaccurate control of the pilot/mains split irrespective of FSVtolerances, variation and friction. The MFFSV position from the LVDT 144is a measure of mains manifold cooling flow during pilot-only operation,and total mains burnt flow during pilot-and-mains operation. This flowmeasurement signal is sent to and used by the EEC control logic toprovide an MFFSV position demand signal that is used to drive the FFSVservo-valve 146 to move the FFSV 135 to set the correct pilot/mains flowsplit (during pilot-and-mains operation) or the correct mains coolingflow (during pilot-only operation).

Although the fuel staging system of US 2016/0273775 addresses many ofthe problems of the system of EP A 2469057, a difficulty arises in thatthe closed loop control of the fuel split using the MFFSV 143 alsoinvolves the EEC. In particular, the digital sample and hold processesof the EEC introduce phase lag in any control loop using the EEC. Thislimits the maximum gain that can be set within the loop, given loopstability considerations, and thus limits response of the closed loop.Typical sample periods for the EEC limit loop bandwidth to 1-2 Hz, whichmay not be fast enough for accurate dynamic control of fuel flow to theengine.

A difficulty in relation to the FSVs is variation in the level ofpiston/sleeve friction acting on individual FSVs. This can result inpoor burner-to-burner flow distribution and thus impacts emissions andturbine life.

The difficulties associated with maldistribution due to a failed openFSV or valve-to-valve frictional variations can be addressed byincreasing the size of the FSVs, as they can then have higher springloads to provide a higher closing force margin and to reduce the impactof piston/sleeve friction on burner-to-burner flow distribution. Moreparticularly, a high spring load requires a large piston diameter inorder to reduce the pressure differential required to open the valve toan acceptable level. Indeed, it can be advantageous to minimise therequired differential so as to limit burner-to-burner maldistribution inmains flow resulting from a stuck open FSV. A further driver for largeFSVs is that pressure losses in the pilot manifold result in differentpilot pressures at the FSVs. Although these losses are typically small,they can be significant relative to the differential pressure requiredto open each valve, and the spring load of all the FSVs must be highenough to ensure that the valve which sees the lowest pilot pressure isable to close and remain closed when de-staging mains. However, thereare significant disadvantages with using large FSVs. They add weight tothe engine, and also may not be feasible to implement if spaceconstraints are tight.

A further difficulty in relation to the FSVs is that, because the pilotflow is routed through the FSV chamber to the pilot side of the pistonfor cooling, reverse purge of the pilots manifold at shutdown can putthe FSV at risk due to back-flushing of contamination from downstreamfilters. In particular, contaminants can degrade FSV regulation as wellas the sealing used within the valve to isolate delivery of fuel to themains combustion zone.

Yet another difficulty in relation to the FSVs is that movement of theFSV pistons during modulation of the staging system can result in adisplacement of fuel between pilot and mains, causing undesired flowtransients (i.e. dips and spikes).

SUMMARY

It would be desirable to provide a combustion staging system thataddresses some or all of these difficulties.

In a first aspect, the present invention provides a combustion stagingsystem for fuel injectors of a multi-stage combustor of a gas turbineengine, the system having:

-   -   a splitting unit which receives a metered total fuel flow and        controllably splits the metered total fuel flow into out-going        pilot and mains fuel flows to perform pilot-only and        pilot-and-mains staging control of the combustor;    -   pilot and mains fuel manifolds which respectively receive the        pilot and mains fuel flows, the mains fuel manifold being split        into a primary line and a servo line such that each line        receives a respective portion of the mains fuel flow;    -   a plurality of mains flow control valves which distribute the        mains fuel flow from the mains fuel manifold to mains discharge        orifices of respective injectors of the combustor, both the        primary line and the servo line extending to the mains flow        control valves before reuniting; and    -   a servo pump operable to change the pressure in the servo line        relative to the pressure in the primary line; and    -   wherein each mains flow control valve has a chamber containing a        movable piston, the chamber to a primary side of the piston        being fed by the primary line, the chamber to a servo side of        the piston being fed by the servo line, the piston being biased        towards a closed pilot-only position which prevents flow out of        the primary side of the chamber to the mains discharge orifice        of the respective injector, and the piston being movable under        an increase in pressure in the servo line relative to the        primary line to an open pilot-and-mains position which allows        flow out of the primary side of the chamber to the mains        discharge orifice of the respective injector.

Advantageously, having the mains flow control valves operated by apressure differential between the servo and primary lines of the mainsfuel manifold allows many of the difficulties associated with the mainsFSVs of US 2016/0273775 to be avoided. In particular, the force marginsfor the control valves can be large, making the system more robust tocontamination and coking, with less sensitivity to variation in valveinternal friction. This reduces the potential for burner-to-burner flowmal-distribution, and thus the potential for degraded emissions andexcessive turbine life reduction. A high servo line pressuredifferential permits a smaller valve and thus eases packaging of thevalve in a burner head of an injector.

In addition, displacement of flow between the pilots and mains whenstaging-in and modulating flows can be reduced as a result of theability to remove pilot flow from the mains flow control valves.

In a second aspect, the present invention provides a fuel supply systemhaving:

-   -   a total fuel metering valve which is configured to receive a        flow of pressurised fuel and to form therefrom a metered total        fuel flow; and    -   a combustion staging system according to the first aspect, the        splitting unit of the combustion staging system receiving the        metered total fuel flow from the fuel metering valve.

The fuel supply system may further have a pressure drop controlarrangement (such as a spill valve and a pressure drop control valve)which is operable to maintain a substantially constant pressure dropacross the total fuel metering valve. The fuel supply system may furtherhave a pressure raising and shut-off valve in the flow path of themetered total fuel flow between the total fuel metering valve and thecombustion staging system.

In a third aspect, the present invention provides a gas turbine enginehaving the fuel supply system of the second aspect.

Optional features of the invention will now be set out. These areapplicable singly or in any combination with any aspect of theinvention.

Typically the piston of the mains flow control valve is spring biasedtowards the closed pilot-only position. For example, the chamber to theservo side of the piston can contain a compression spring biasing thepiston towards the closed pilot-only position.

The servo pump can be a positive displacement pump, such as a gear pumpetc. Another option, however, is for the servo pump to be a roto-dynamicpump, such as a centrifugal pump.

The servo pump can be electrically, hydraulically (e.g. by fuel or otherliquid) or pneumatically powered.

The primary line and the servo line may conveniently reunite downstreamof a back pressure orifice located in the servo line to maintain thechanged pressure in the servo line relative to the pressure in theprimary line.

The combustion staging system may further have a relief valve connectedacross the servo pump to relieve excess pressure in the servo line e.g.in the case of blockage at the back pressure orifice. Such a reliefvalve may be particularly desirable when the servo pump is a positivedisplacement pump. However, if the servo pump is electrically poweredanother option for relieving excess pressure in the servo line is toprovide a limit on the power supplied to the pump at a given pump speed.

Advantageously, the mains flow control valves may be binary operatingvalves which are either fully open or fully closed. This cansubstantially improve the operation and reliability of the valves. Also,from an operation perspective, if a binary operating valve fails open,it is then in the same state as a normally open valve, thus avoiding thepotential for burner-to-burner flow mal-distribution when mains isstaged-in, decreasing the potential for excessive turbine lifereduction.

Conveniently, the mains flow control valves can have single face seals.This is because there is no need for a high degree of sealing betweenthe primary and servo lines (i.e. a degree of leakage between theselines is acceptable). In contrast, the FSVs of US 2016/0273775 havedual-face seals to provide a drip tight seal to stop delivery of flow tothe mains combustion process when mains is de-staged. Such dual-faceseals are intrinsically less reliable and robust than single face seals.

The splitting unit can include a fuel flow splitting valve, e.g. of thetype disclosed in US 2016/0273775. Thus such a valve can have acontrollably slidable spool, the position of which determines a split ofthe metered total fuel flow into the valve between out-going pilot andmains fuel flows from the valve. A position sensor, such as an LVDT, onthe fuel flow splitting valve can measure the position of the spool toindicate the pilot/main split. Additionally or alternatively, thesplitting unit may further include a fuel flow sensing valve downstreamof the fuel flow splitting valve to sense the out-going pilot or mainsfuel flow.

However, another option is for the splitting unit to implement ametering and spill architecture. In this case, the splitting unit mayinclude a metering valve and a (further) spill valve, a first portion ofthe total metered fuel flow received by the splitting unit being aninflow to the metering valve and a second portion of the total meteredfuel flow received by the splitting unit being an inflow to the spillvalve, the metering valve being configured to controllably determine afuel flow rate of a metered outflow formed from the first portion of thetotal metered fuel flow, the spill valve being configured to produce aspilled outflow formed from the second portion of the total metered fuelflow, and the spill valve being further configured to sense a pressuredifferential between the inflow to and the metered outflow from themetering valve and to vary the amount of the spilled outflow in responseto the sensed pressure differential, whereby the metered outflow formsone of the pilot and mains fuel flows, and the spilled outflow forms theother of the pilot and mains fuel flows. Advantageously, the meteringvalve and the spill valve of the splitting unit can provide purelyhydro-mechanical closed loop control of the fuel split. The higherbandwidth of this form of control facilitates more accurate dynamiccontrol of fuel flow to the engine. The use of such a splittingarchitecture can also reduce the sensitivity of the fuel split tochanges in the flow characteristics of downstream components, such asthe burner nozzles, which may block progressively over time.

When the splitting unit implements the metering and spill architecture,the splitting unit may further include a mains throttle valve whichthrottles the out-going mains fuel flow from the splitting unit inresponse to the pressure of the out-going pilot fuel flow from thesplitting unit. In contrast, the FSVs of US 2016/0273775 throttle themains flow therethrough in response to the pressure of the pilot flow.The mains throttle valve can displace flow in a similar way to the FSVsof US 2016/0273775, but being a single valve its net displacement areacan be significantly smaller. This can decrease dynamic cross-talkbetween the pilot and mains flows, thereby reducing undesirable flowtransients.

Preferably, when the splitting unit implements the metering and spillarchitecture, the metered outflow forms the mains fuel flow, and thespilled outflow forms the pilot fuel flow. This allows the mains fuelflow to be controlled more accurately, which can be particularlybeneficial for control of a cooling flow (discussed below) when mains isde-staged. This does not exclude, however, that the metered outflow canform the pilot fuel flow, and the spilled outflow can form the mainsfuel flow.

When the splitting unit implements the metering and spill architecture,the metering valve may have a spool whose position is controllable todetermine the fuel flow rate of the metered outflow. For example, theposition of the spool may be controlled by a servo-valve, e.g. under thecommand of the engine electronic controller. The metering valve mayfurther have a device to measure the position of the spool. Thisposition measurement can be fed to the engine electronic controller foruse in controlling the spool position.

Irrespective of its architecture, the splitting unit may send a coolingflow to the mains fuel manifold during pilot-only operation. In thiscase, the system may further have a mains cooling valve which, duringpilot-only operation, opens a bypass connection between the mains andpilot fuel manifolds such that the cooling flow passes from the mainsfuel manifold to the pilot fuel manifold. The cooling flow through themains fuel manifold in the pilot-only mode then helps to avoid coking inthe mains manifold when the mains flow control valves are in theirpilot-only positions. Advantageously, the cooling flow can pass to pilotdischarge orifices of the injectors for burning in the combustor,avoiding the need for any kind of recirculation architecture and therebyreducing fuel heating and residence time so that the possibility of fuelbreakdown is reduced. Preferably, the mains cooling valve ishydraulically operated by the pressure in the servo line relative to thepressure in the primary line. For example, when the servo pump operatesto increase the pressure in the servo line relative to the pressure inthe primary line, the increased pressure in the servo line may close themains cooling valve such that the cooling flow passing from the mainsfuel manifold to the pilot fuel manifold is interrupted.

When the combustion staging system has the mains cooling valve, thesystem may further have a non-return valve in the bypass connectionwhich prevents flow in the direction from the pilot fuel manifold to themains fuel manifold. The non-return valve may further have a device,such as an LVDT, to measure flow through the non-return valve. Thismeasurement can be used to identify a failed open mains flow controlvalve when mains is de-staged. This helps avoid a need for expensive,complex and potentially unreliable thermocouple-based systems to detecta failed open FSV.

When the splitting unit implements the metering and spill architecture,according to one option for the cooling flow, the spill valve of thesplitting unit can send some or all of the cooling flow to the mainsfuel manifold during pilot-only operation. In particular, the spillvalve may receive a third portion of the total metered fuel flow (e.g.through a fixed servo orifice) and may form some or all of the coolingflow therefrom. Additionally or alternatively, (particularly applicablewhen the metered outflow forms the mains fuel flow) the metering valvemay send some or all of the cooling flow to the mains fuel manifoldduring pilot-only operation.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

FIG. 1 shows schematically a known staging system for fuel injectors ofthe combustor of a gas turbine engine of FIG. 1 in pilot-only operatingmode;

FIG. 2 shows a longitudinal cross-section through a ducted fan gasturbine engine;

FIG. 3 shows schematically a staging system for fuel injectors of thecombustor of the engine of FIG. 2 in pilot+mains operation mode; and

FIG. 4 shows schematically the staging system of FIG. 3 in pilot-onlyoperation mode.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES

With reference to FIG. 2, a ducted fan gas turbine engine incorporatingthe invention is generally indicated at 10 and has a principal androtational axis X-X. The engine comprises, in axial flow series, an airintake 11, a propulsive fan 12, an intermediate pressure compressor 13,a high-pressure compressor 14, combustion equipment 15, a high-pressureturbine 16, an intermediate pressure turbine 17, a low-pressure turbine18 and a core engine exhaust nozzle 19. A nacelle 21 generally surroundsthe engine 10 and defines the intake 11, a bypass duct 22 and a bypassexhaust nozzle 23.

During operation, air entering the intake 11 is accelerated by the fan12 to produce two air flows: a first air flow A into theintermediate-pressure compressor 13 and a second air flow B which passesthrough the bypass duct 22 to provide propulsive thrust. Theintermediate-pressure compressor 13 compresses the air flow A directedinto it before delivering that air to the high-pressure compressor 14where further compression takes place.

The compressed air exhausted from the high-pressure compressor 14 isdirected into the combustion equipment 15 where it is mixed with fueland the mixture combusted. The resultant hot combustion products thenexpand through, and thereby drive the high, intermediate andlow-pressure turbines 16, 17, 18 before being exhausted through thenozzle 19 to provide additional propulsive thrust. The high,intermediate and low-pressure turbines respectively drive the high andintermediate-pressure compressors 14, 13 and the fan 12 by suitableinterconnecting shafts. Although FIG. 2 shows a three spool turbofanengine, the present invention is equally applicable to other enginearchitectures, such as two or single spool engines, and/or geared fanengines.

The engine has a pumping unit comprising a low pressure (LP) pumpingstage which draws fuel from a fuel tank of the aircraft and supplies thefuel at boosted pressure to the inlet of a high pressure (HP) pumpingstage. The LP stage typically comprises a centrifugal impeller pumpwhile the HP pumping stage may comprise one or more positivedisplacement pumps, e.g. in the form of twin pinion gear pumps. The LPand HP stages are typically connected to a common drive input, which isdriven by the engine HP or IP shaft via an engine accessory gearbox.

A fuel supply system then accepts fuel from the HP pumping stage forfeeds to the combustor 15 of the engine 10. This system typically has ahydro-mechanical unit (HMU) comprising a fuel metering valve operable tocontrol the rate at which fuel is allowed to flow to the combustor. TheHMU may further comprise a pressure drop control arrangement (such as aspill valve and a pressure drop control valve) which is operable tomaintain a substantially constant pressure drop across the meteringvalve, and a pressure raising and shut-off valve at the fuel exit of theHMU which ensures that a predetermined minimum pressure level ismaintained upstream thereof in a filtered servo flow for correctoperation of any fuel pressure operated auxiliary devices (such variableinlet guide vane or variable stator vane actuators) that receive fuelunder pressure from the HMU. Further details of such an HMU aredescribed in EP A 2339147.

An engine electronic controller (EEC—not shown) commands the HMU fuelmetering valve to supply fuel at a given flow rate to a staging system30 (shown schematically in FIG. 3 in pilot+mains operation mode, and inFIG. 4 in pilot-only operation mode) and thence to fuel injectors 34 ofthe combustor 15. The metered total fuel flow leaves the HMU and arrivesat the staging system at a pressure P_(fmu).

Parts of the staging system 30 are similar or identical to thecorresponding parts of the known system 130 shown in FIG. 1. Thus thestaging system 30 splits the fuel under the control of the EEC into twoflows: one for first 31 and second 32 pilot manifolds and the other fora mains manifold 33. The first pilot manifold feeds pilot dischargeorifices of a subset of the fuel injectors (via respective weightdistribution valves—WDVs). The second pilot manifold feeds pilotdischarge orifices of the rest of the fuel injectors (also viarespective WDVs). The mains manifold feeds mains discharge orifices ofall the fuel injectors. A splitting unit 50 (described in more detailbelow) receives the metered total fuel flow from the HMU and produces anoutgoing flow split between a pilot connection pipe 36 which deliversfuel to the first 31 and second 32 pilot manifolds and a mainsconnection pipe 37 which delivers fuel to the mains manifold 33. Thesecond pilot manifold 32 connects to the pilot connection pipe 36 via afurther connection pipe 39 and a lean blow out protection valve 41controlled by way of a solenoid-operated control valve 42. The splittingunit also sends a cooling flow to the mains manifold during pilot-onlyoperation.

The system 30 has a mains cooling valve 47 which in pilot-only operationopens a bypass connection between the mains manifold 33 and theconnection pipe 36, allowing the cooling flow sent to the mains manifold33 during pilot-only operation to pass from the mains fuel manifold tothe pilot fuel manifolds 31, 32, and thence onwards for burning at thepilot orifices of the injectors 34. The mains cooling valve 47 closesduring pilot-and-mains operation.

A key difference between the staging system 30 shown in FIGS. 3 and 4and the known system 130 shown in FIG. 1 is that mains flow controlvalves 40 are provided at the injectors 34 rather than mains FSVs.Moreover, the mains fuel manifold 33 is split into a primary line 33 aand a servo line 33 b which each receive a respective portion of themains fuel flow. The mains flow control valves 40 distribute the mainsflow from the primary line to the mains discharge orifices in theinjectors 34 via respective WDVs, and the operation of the mains flowcontrol valves is controlled by a pressure differential between theprimary and the servo lines. This arrangement is discussed further belowafter a more detailed discussion of the splitting unit 50.

A further difference between the staging system 30 shown in FIGS. 3 and4 and the known system 130 shown in FIG. 1 is that the splitting unit 50has a fuel flow metering and spill architecture rather than a fuel flowsplitting valve and a fuel flow sensing valve. However, although themetering and spill architecture provides advantages, which are discussedbelow, an architecture based on a fuel flow splitting valve could beimplemented instead.

More particularly, the splitting unit 50 has a staging metering valve(SMV) 51 providing a variable metering orifice with a known, andaccurately controlled, relationship between area and metering spoolposition. Control of the position of the spool of the SMV allows controlof the metering orifice area. The position of the metering piston ismeasured using a position sensor, such as an LVDT 53, and its positionis controlled using a two stage servo-valve (MSV) 54. A spill valve(SSV) 52 of the splitting unit 50 controls the pressure differential setacross the metering orifice such that control of metering valve positiongives accurate control of flow delivered by the SMV into the mainsconnection pipe 37, this flow being the mains fuel flow sent to themains manifold 33.

However, the fuel flow delivered through the SMV 51 is only a firstportion of the HMU total metered fuel flow received by the splittingunit 50. A second portion of the received total metered fuel flow passesthrough a staging spill valve (SSV) 52 into the pilot connection pipe 36to form the pilot fuel flow. The SSV can be a two-stage valve, with apilot (first) stage of the SSV sensing the pressure differential setacross the SMV and varying the position of a second stage piston to varythe area of the spill profile in a valve sleeve. Similarly, the SSV canbe a single stage valve with a single piston both sensing the pressuredifferential set across the SMV and moving to vary the area of the spillprofile. Opening the spill profile of the SSV permits more spill flow topass to the pilot connection pipe and thus reduces the mains fuel flowfrom the SMV (as the mains flow+the pilot flow=HMU total metered fuelflow). The converse is true for SSV closure.

An advantage of the fuel flow metering and spill architecture of thesplitting unit 50 is that control of the pressure drop across the SMV 51can be achieved hydro-mechanically and is therefore capable of asignificantly higher bandwidth than can be achieved with the controlarrangement outlined in US 2016/0273775. More accurate control of mainsflow during transients can thus be achieved, such as when the meteredtotal fuel flow is changing, flow split is changing, or mains is beingstaged-in or out. Transient dips and spikes in fuel flow risk enginesurge or flameout, so their reduction is important.

The SMV 51 is used to meter flow to the mains connection pipe 37 whenmains is both staged-in or staged-out. In the latter case, a residualmetered flow from the SMV can be used to form the cooling flow sent tothe mains manifold 33 during pilot-only operation. For example, when thespool of the SMV moves into a position corresponding to pilot-onlyoperation (FIG. 4) it could open an additional port on the SMV (notshown in FIG. 4) to open the cooling flow path. However, as shown inFIG. 4, another option is for the two-stage SSV 52 to meter a fixed flowin parallel to the SMV metering orifice. This flow is formed from athird portion of the received total metered fuel flow and is taken froma flow washed filter (SFWF) 56 at the inlet to the splitting unit 50. Itthen passes through a fixed servo orifice (SSO) 55 before passingthrough a variable poppet orifice within the SSV into the mainsconnection pipe downstream of the SMV. The rate of this fixed flow canbe aligned with the cooling flow required to cool the mains manifold,allowing the SMV to be fully closed in pilot-only operation. Thisarrangement advantageously reduces the risk of excessive cooling flowsresulting from any SMV control problems. In particular, such excessivecooling flows can increase the risk of undesired opening of the mainsflow control valves 40. The two-stage SSV is also more robust than asingle-stage spill valve to fuel borne contamination and coking, andthus provides better control of flow splitting between pilots and mains,which in turns offers better control of engine emissions. However, thisdoes not exclude that the splitting unit could use a single-stage SSVinstead of the two-stage SSV shown.

The metering and spill architecture of the splitting unit 50 is madepossible by the need to maintain flow in the both the pilot manifolds31, 32 and the mains manifold 33 when the engine is running. Inpilots-only mode of operation, cooling flow is metered into the mainsmanifold for cooling purposes and re-joins the pilots burnt flow streamvia the mains cooling valve 47.

A non-return valve 57 can be added to the bypass connection controlledby the mains cooling valve 47 between the pilot manifolds 31, 32 and themains manifold 33. The non-return valve accommodates a scenario whereone of the mains flow control valves 40 has failed open when mains isstaged-out. Without the non-return valve it would be possible for pilotflow to pass to the mains manifold. This flow would increase as thepressure differential across the pilot discharge orifices of the fuelinjectors increases. Passing to the mains combustion zone through thefailed mains flow control valve, the flow could result in localisedheating of turbine components, leading to a reduction of turbine lifeand possible turbine failure.

The operation of the mains cooling valve 47 and the non-return valve 57is explained further below as part of the discussion of the mains flowcontrol valves 40 and the split mains fuel manifold 33.

As mentioned above, the staging system 30 includes a lean blow outprotection valve (LBOV) 41 controlled by way of a solenoid-operatedcontrol valve (LBSV) 42. The high pressure feed for the LBSV can beconfigured to be taken from the SFWF 56 at the inlet to the splittingunit 50. Any leakage flow from this feed then returns to the secondpilot manifold 32 via the connection pipe 39. Benefits of such a servosupply for the LBOV are:

-   -   1. The leakage into the second pilot manifold 32 when the engine        is running is part of the HMU metered total fuel flow, reducing        the potential for delivery of incorrect flow levels to the        engine.    -   2. When the engine is shut-down, fuel cannot leak into the        staging system 30 via the LBOV 41. In the staging system 130 of        FIG. 1, such leakage from HPf is possible if the seal of the        solenoid operated control valve 142 is not drip tight when its        plunger is in the position shown in FIG. 1.

It would be possible to configure the splitting unit 50 such that themetered flow stream from the SMV 51 is directed to the pilot connectionpipe 36 to form the pilot fuel flow and the mains flow is formed fromthe remaining spill through the SSV 52. However, in this case the spillthrough the SSV 52 would need to be controlled quite accurately toproduce the cooling flow when mains is staged-out. Metering the pilotflow means that the residual mains flow is the difference between theHMU metered total flow and metered pilot flow. Inaccuracies in meteringeither of these flows could result in too wide a range of cooling flows.In particular, too low a cooling flow could cause excessive fueltemperatures in the de-staged mains line, while too high a cooling flowcould risk opening the FSVs 40 as a result of an excessive pressuredifferential between mains and pilot when mains is de-staged.

We turn next to consideration of the mains flow control valves 40 andthe split mains fuel manifold 33. Conveniently, the mains flow controlvalves can be binary operated valves which permit or stop the deliveryof metered mains flow to the mains combustion zone for burning. Closureof the mains flow control valves stops the mains flow to the combustor(de-staging), while opening of the valves allows mains flow (staging).Binary operation is caused by application or removal of a pressuredifferential applied to each binary valve and generated by a stagingservo pump (SSP) 58 located in the servo line 33 b of the mains fuelmanifold and powered by a motor 59. In FIGS. 3 and 4, the motor is shownas an electrical motor, but this does not exclude that it can be ahydraulic (e.g. using fuel or other engine fluid) or a pneumatic motor.

As shown in FIGS. 3 and 4, the mains fuel manifold is split into theprimary 33 a and servo 33 b lines at a cooling flow washed filter (CFWF)63. The SSP 58 can be a gear-type positive displacement pump which drawsflow through the CFWF into the servo line. Both the primary and theservo lines extend through each mains flow control valve 40 in series,before reuniting downstream of a back pressure orifice (BPO) 61. Thusthe portion of the mains flow diverted through the servo line ultimatelyreturns to the primary line for burning. The BPO provides a restrictionto flow that generates a pressure differential across the SSP.

The mains flow control valves 40 each have a chamber containing amovable, spring-biased piston, with the chamber to a servo (spring) sideof the piston being in fluid communication with the servo line 33 b andthe chamber to a primary (non-spring) side of the piston being in fluidcommunication with the primary line 33 a. With a faster pump speed ofthe SSP 58, a higher pressure differential can be generated across thepistons. A high pressure differential overcomes a closing spring biasacting on the pistons, such that the valves open for staging. Removal ofthe differential allows the spring bias to close the valves.

Advantageously, the mains flow control valves 40 can have single faceseals, rather than the dual face seals of the FSVs 140 of the system ofFIG. 1, as some leakage between the primary 33 a and servo 33 b lines inthe valves is acceptable.

As the mains flow control valves 40 are binary operated valves which donot rely on the pressure of the pilot flow, they do not allow the pilotflow to throttle the mains flow in the manner of the FSVs 140 of thesystem of FIG. 1 and without this throttling function it can bedifficult to achieve the required range of pilot to mains flow splits.Accordingly, a mains throttle valve 62 can be provided in the splittingunit 50 to displace flow in a similar way to the FSVs 140. However, asthe mains throttle valve is a single valve its net displacement area canbe significantly smaller than that of the FSVs 140, which can help toreduce dynamic cross-talk between the pilot and mains flows.

The velocity of opening of the mains flow control valves 40 isdetermined by: the force balance on their pistons, the size of therestriction of the BPO 61 and the flow made available from the SSP 58.In contrast, the velocity of closing of the mains flow control valves isdetermined from their force balance and the size of the BPO restrictionalone, since the pump speed is reduced (e.g. to zero) when de-staging.Advantageously, the system can effect transition of the mains flowcontrol valves from one end to another in around one second.

Although the SSP 58 is illustrated in FIGS. 3 and 4 as a gear pump, anytype of positive displacement pump can be used to generate the servodifferential pressure. As another option, a roto-dynamic pump, such as acentrifugal pump, could be used to generate the differential pressure.

The binary operating mains flow control valves 40 can improve therobustness of the staging system 30, and reduce the possibility ofundesirable operation. In particular, one of the problems with FSVs 140shown in FIG. 1, is the effect of one FSV failing open. At low meteredflows, a tendency is for the majority of the flow to pass through thefailed-open valve. The remainder of the FSVs would be less open orclosed and pass less flow, creating a local hot spot in the engine,which can elevate turbine temperature to the point where turbineintegrity is lost or turbine life is significantly reduced. This can beavoided, however, in the staging system 30 of FIGS. 3 and 4, by havingthe normally open position of the mains flow control valves 40 equal tothe position of a failed-open valve.

A further benefit of the binary operating mains flow control valves 40is their reduced sensitivity to variation in friction (from differencesin build, wear, and debris contamination) acting on the piston of eachvalve. For the FSVs 140 shown in FIG. 1, this variation results indegradation in flow split between burners, affecting combustionemissions and potentially creating turbine hot spots as above. The mainsflow control valves 40, by contrast, can tolerate significantly higherfrictional loads before their operation is affected.

After completion of de-staging, the SSP 58 can be run at a slow speedsuch that some of the cooling flow generated by the splitting unit 50(as discussed above) is sent through the servo line 33 b as well asthrough the primary line 33 a. The relative amounts of the cooling flowsthrough the lines can be managed by appropriate control of pump speedinformed by measurement of servo pressure differential between lines 33b and 33 a or flow rate in the servo line 33 b.

A position sensor, e.g. in the form of an LVDT 64, on the non-returnvalve 57 can be used to provide flow measurement and thereby identifythe occurrence of a mains flow control valve 40 that has failed in anopen position when required to be closed. The EEC can then take suitableaction to end the delivery of the cooling flow by ceasing the deliveryof total flow to the staging unit or staging in mains to maintaindelivery of engine power, albeit at the expense undesired engineemissions. More particularly, if a mains flow control valve fails in anopen position when it should be closed, the subsequent leakage ofcooling flow to the combustion process results in a lower pressure inthe mains fuel manifold 33 than in the pilot fuel manifolds 31, 32. Thisthen causes the non-return valve to close when it should be open,thereby announcing the failure.

The mains cooling valve 147 in the system of FIG. 1 is solenoid operatedand located relatively close to the injectors 134. In contrast, themains cooling valve 47 in the staging system 30 of FIGS. 3 and 4, iscloser to the splitting unit 50 and is hydraulically operated on thebasis of the pressure differential between the primary 33 a and theservo 33 b lines. These changes improve the robustness of the mainscooling valve. Firstly, the location of the valve allows its relativelysmall cooling orifice to reside in an environment where temperature canbe better controlled, reducing a risk of blockage due to build-up offuel breakdown products. Secondly, the hydraulic operation allows thevalve to operate with larger forces than solenoid operation, enabling amore robust valve structure.

A relief valve 60 connected across the SSP 58 can be provided to limitpressures in the servo line 33 b in the event of blockage of the BPO 61.However, this relief function can be provided through limitation ofinput power if an electric drive is used to drive the SSP and the pumpspeed is measured.

Further variants of the combustion staging system shown in FIGS. 3 and 4are possible. For example:

-   -   1) The mains flow control valves 40 can be configured to operate        at different pressure differentials so as to provide a means of        staging mains. As an example, half of the total number of the        valves could use a higher spring load than the other half, such        that operation of the SSP 58 could be used to open half the        valves before the remainder open. This increases the minimum        flow passing through any one mains flow control valve to the        corresponding mains discharge orifice, thus avoiding potential        issues of degraded operation resulting from undesirable flow        regimes within the nozzle assembly of the injectors at low        flows.    -   2) A differential pressure sensor could be provided to sense the        pressure differential generated by the SSP 58 such that closed        loop control can be used to more accurately regulate the        differential generated by the SSP. The loop could be closed by        the EEC, with pump speed being varied to change delivered        differential. Accurate regulation of multiple pressure levels        could be beneficial to achieve mains staging.    -   3) Sensing of the differential pressure between the mains and        pilot flows can be used to diagnose a mains flow control valve        40 in a failed open state when mains is staged-out. Such        pressure sensing can be an alternative to the measurement        provided by the LVDT 64 of the non-return valve 57.    -   4) The staging system could use a splitting unit based on a fuel        flow splitting valve (FFSV) of the type shown in FIG. 1 in place        of the metering and spill architecture of the splitting unit 50        of FIGS. 3 and 4.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

All references referred to above are hereby incorporated by reference.

What is claimed is:
 1. A combustion staging system for fuel injectors ofa multi-stage combustor of a gas turbine engine, the system having: asplitting unit which receives a metered total fuel flow and controllablysplits the metered total fuel flow into out-going pilot and mains fuelflows to perform pilot-only and pilot-and-mains staging control of thecombustor; pilot and mains fuel manifolds which respectively receive thepilot and mains fuel flows, the mains fuel manifold being split into aprimary line and a servo line such that each line receives a respectiveportion of the mains fuel flow; a plurality of mains flow control valveswhich distribute the mains fuel flow from the mains fuel manifold tomains discharge orifices of respective injectors of the combustor, boththe primary line and the servo line extending to the mains flow controlvalves before reuniting; and a servo pump operable to change thepressure in the servo line relative to the pressure in the primary line;and wherein each mains flow control valve has a chamber containing amovable piston, the chamber to a primary side of the piston being fed bythe primary line, the chamber to a servo side of the piston being fed bythe servo line, the piston being biased towards a closed pilot-onlyposition which prevents flow out of the primary side of the chamber tothe mains discharge orifice of the respective injector, and the pistonbeing movable under an increase in pressure in the servo line relativeto the primary line to an open pilot-and-mains position which allowsflow out of the primary side of the chamber to the mains dischargeorifice of the respective injector.
 2. A combustion staging systemaccording to claim 1, wherein the primary line and the servo linereunite downstream of a back pressure orifice located in the servo lineto maintain the changed pressure in the servo line relative to thepressure in the primary line.
 3. A combustion staging system accordingto claim 1, wherein the mains flow control valves are binary operatingvalves which are either fully open or fully closed.
 4. A combustionstaging system according to claim 1, wherein the splitting unit includesa metering valve and a spill valve, a first portion of the total meteredfuel flow received by the splitting unit being an inflow to the meteringvalve and a second portion of the total metered fuel flow received bythe splitting unit being an inflow to the spill valve, the meteringvalve being configured to controllably determine a fuel flow rate of ametered outflow formed from the first portion of the total metered fuelflow, the spill valve being configured to produce a spilled outflowformed from the second portion of the total metered fuel flow, and thespill valve being further configured to sense a pressure differentialbetween the inflow to and the metered outflow from the metering valveand to vary the amount of the spilled outflow in response to the sensedpressure differential, whereby the metered outflow forms one of thepilot and mains fuel flows, and the spilled outflow forms the other ofthe pilot and mains fuel flows.
 5. A combustion staging system accordingto claim 4, wherein the splitting unit includes a mains throttle valvewhich throttles the out-going mains fuel flow from the splitting unit inresponse to the pressure of the out-going pilot fuel flow from thesplitting unit.
 6. A combustion staging system according to claim 4,wherein the metered outflow forms the mains fuel flow, and the spilledoutflow forms the pilot fuel flow.
 7. A combustion staging systemaccording to claim 4, wherein the metering valve has a spool whoseposition is controllable to determine the fuel flow rate of the meteredoutflow.
 8. A combustion staging system according to claim 7, whereinthe metering valve further has a device to measure the position of thespool.
 9. A combustion staging system according to claim 1, wherein thesplitting unit sends a cooling flow to the mains fuel manifold duringpilot-only operation; and wherein the system further has a mains coolingvalve which, during pilot-only operation, opens a bypass connectionbetween the mains and pilot fuel manifolds such that the cooling flowpasses from the mains fuel manifold to the pilot fuel manifold.
 10. Acombustion staging system according to claim 9, wherein the systemfurther has a non-return valve in the bypass connection which preventsflow in the direction from the pilot fuel manifold to the mains fuelmanifold.
 11. A combustion staging system according to claim 10, whereinthe non-return valve further has a device to measure flow through thenon-return valve.
 12. A combustion staging system according to claim 9,wherein the splitting unit includes a metering valve and a spill valve,a first portion of the total metered fuel flow received by the splittingunit being an inflow to the metering valve and a second portion of thetotal metered fuel flow received by the splitting unit being an inflowto the spill valve, the metering valve being configured to controllablydetermine a fuel flow rate of a metered outflow formed from the firstportion of the total metered fuel flow, the spill valve being configuredto produce a spilled outflow formed from the second portion of the totalmetered fuel flow, and the spill valve being further configured to sensea pressure differential between the inflow to and the metered outflowfrom the metering valve and to vary the amount of the spilled outflow inresponse to the sensed pressure differential, whereby the meteredoutflow forms one of the pilot and mains fuel flows, and the spilledoutflow forms the other of the pilot and mains fuel flows; and whereinthe spill valve sends some or all of the cooling flow to the mains fuelmanifold during pilot-only operation.
 13. A combustion staging systemaccording to claim 12, wherein the spill valve receives a third portionof the total metered fuel flow and forms some or all of the cooling flowtherefrom.
 14. A fuel supply system having: a fuel metering valve whichis configured to receive a flow of pressurised fuel and to formtherefrom a metered total fuel flow; and a combustion staging systemaccording to claim 1, the splitting unit of the combustion stagingsystem receiving the metered total fuel flow from the fuel meteringvalve.
 15. A gas turbine engine having the fuel supply system accordingto claim 14.